Normally, it is known that by forming a thin film or injecting ions in plasma onto the surface of a solid material, the solid surface characteristics are improved. A film formed using plasma including metal ions and nonmetal ions strengthens the abrasion and corrosion resistances of a solid surface, and it is useful as a protective film, an optical thin film, and a transparent electroconductive film among others. In particular, as for carbon films using carbon plasma, the utility value is high as diamond like carbon films (so-called DLC films) comprising amorphous mixed crystals of diamond and graphite structures,
As a method for generating plasma including metal ions and nonmetal ions, there is a vacuum arc plasma method. Vacuum arc plasma is formed by an arc discharge occurring between a cathode and an anode. The cathode material evaporates from an existing cathode spot of the cathode surface, and it is plasma formed by this vaporized cathode material. Also, when a reactive gas is introduced as the environmental gas, the reactive gas is ionized simultaneously. An inert gas (so-called noble gas) may be introduced together with said reactive gas, and said inert gas can also be introduced instead of said reactive gas. By means of such plasma, a surface treatment can be done by a thin film formation or an ion injection onto a solid surface.
Normally, in a vacuum arc discharge, at the same time as vacuum arc plasma constituent particles such as cathode material ions, electrons, and cathode material neutral atom groups (atoms and molecules) are ejected by a cathode spot, cathode material particles named droplets of size ranging from less than submicron to several hundred microns (0.01-1000 μm) are also ejected. However, it is generation of droplets that cause problems in a surface treatment such as a film formation. When these droplets adhere to the surface of an object to be treated, the uniformity of a film formed on the surface of the object to be treated surface is lost, and a defective thin film is caused.
As one method to solve the problem of droplets, there is a magnetic filter method. As an example of this magnetic filter method, there is one in which vacuum are plasma is transported to the plasma processing portion through a curved droplet capture duct. According to this method, the generated droplets are adhered and captured (collected) at the duct inner peripheral wall, and a plasma stream that nearly does not contain droplets is obtained at the duct outlet. Also, a curved magnetic field is formed by magnets placed along the duct, and it is comprised so that the plasma stream is bent by this curve magnetic field, and the plasma is guided efficiently to the plasma processing portion.
A plasma arc machining apparatus having a droplet collecting portion is disclosed in Japanese Patent Laid-Open No. 2002-8893 bulletin (patent document 1). FIG. 15 is a schematic configuration diagram of a conventional plasma processing apparatus. At plasma generating portion 102, an electric spark is caused between cathode 104 and trigger electrode 106, and plasma 109 is produced by generating a vacuum arc between cathode 104 and anode 108. Power supply 110 for generating an electric spark and a vacuum arc discharge is connected to plasma generating portion 102, and plasma stabilizing magnetic field generators 116a, 116b for stabilizing plasma 109 are positioned. Plasma 109 is guided to plasma processing portion 112 from plasma generating portion 102, and object to be treated 114 placed in plasma processing portion 112 is surface-treated by said plasma 109. Also, a reactive gas is introduced as necessary by gas introduction system Gt connected to plasma processing portion 112, and reactant gases and the plasma stream are exhausted by gas exhaust system Gh.
Plasma 109 ejected from plasma generating portion 102 is bent to a T-shape toward a direction away from plasma generating portion 102 by the magnetic field, and is flowed into plasma processing portion 112. At the position facing plasma generating portion 102, droplet collecting portion 120 is positioned, where cathode material particles (droplets) 118 generated as a byproduct at cathode at the time of generation of plasma 109 are collected. Therefore, droplets 118 not under an influence of the magnetic field advances to droplet collecting portion 120 and are collected, thereby preventing an intrusion of droplets 118 into plasma processing portion 112. As for a specific droplet collecting means, for example, in Japanese Patent Laid-Open No. 2002-105628 bulletin (patent document 2), it is disclosed that droplets that do not arrive at a plasma processing portion are adhered and collected by baffles set up in a plasma duct inner wall,
[patent document 1] Japanese Patent Laid-Open No. 2002-8893 bulletin
[patent document 2] Japanese Patent Laid-Open No. 2002-105628 bulletin